catalina island is great for a day trip. North channel islands san miguel, anacapa, etc are great for mutiday and day trips . Check out truth aquatics web site for scedules. They are a great operation. Beware water is COLD!
Seconding the Truth boats. truthaquatics.com. They are based in Santa Barbara.
Geography-wise you have:
Northern CA (North of Santa Barbara) has some good shore diving, but I'm not familiar with the particulars, somebody else will no doubt chime in on that.
As far as Southern CA, there are a few options:
Channel Islands are made up of the "Northern" channel islands (Anacapa, Santa Cruz, Santa Rosa and San Miguel) - from closest in to farthest out. These can all be reached on day boats, but the swell can often interfere with getting all the way to Santa Rosa or San Miguel, so you have a medium chance of ending up at Santa Cruz Island (not to be confused with Santa Cruz city) instead. Santa Cruz island is pretty so-so IMO, I wouldn't head there as the intended destination. You can still find neat stuff though if you end up there because conditions are bad. Santa Rosa and San Miguel, of course, being the farthest away, are the best ones! Think pinnacles absolutely covered with invertibrate life. Strawberry anemones, yellow cup corals, nudis all over the place, rockfish, lingcod, lobsters, barnacles (surprisingly cute and photogenic). The Truth boats (TruthAquatics.com) and the Peace Boat (I think it's peacediveboat.com) are the most popular options for getting out there. The Truth boats are cushier by alot, but the Peace has a huge following. The trips to the "outer islands" (as they call them) usually board the night before and sleep on the boat, as they leave at 1am or 4am (depending on the destination). Sleeping arrangements are bunk room style, byo sleeping bag and earplugs.
Anacapa is the easiest of the northern channel islands to get to, and there are a few boats that do easy and fun day trips out there. Most leave from Ventura Harbor. I like the Spectre the best, but that's my own personal opinion, there are plenty of boats from 6 packs on up. The Spectre goes pretty much every day, and typically does 4 dives at various spots on Anacapa. You can contact them at CalBoatDiving.com. At Anacapa you'll almost certainly get one dive that drops you over a shallow sandy patch where all you do is play with sea lions the whole dive. It's a blast. Anacapa has "typical" CA shallow diving - mostly shallower than 60'. There are a few pinnacles that the boats may take you to if the conditions are good. About half the dives tend to be in marine parks rich with kelp, garibaldis, sheephead, etc. Cathedral Cove, which they usually try to stop at every day, can be stunning, especially if you want to fiddle around in shallow water without (too much) surge trying to take pictures of kelp and nudis and sea lions.
Southern Channel Islands include Catalina, Santa Barbara and San Clemente (as well as the remote areas like Cortes Banks). These are served by day boats out of Long Beach/San Pedro, Ventura sometimes (I think the Peace comes south?) and the Redondo Beach area. Catalina is also serviced by the ferry (catalinaflyer.com), but alot of people prefer to dive from boats, as the shore diving on Catalina is pretty much just Casino Point (the marine park, which is beautiful kelp forest diving). It's kind of a schlep with the gear unless you rent on the island, which I would recommend. Somebody can give you the name of the main dive shop there - ScubaLuv? Also have an island based boat. The boats reach some neat spots like Italian Gardens (known for giant sea bass), a couple of wrecks, etc. One dive that is kind of a highlight is Farnsworth Bank, which is a "destination" (the boat will advertise that as the destination for the day). It's a deep pinnacle (top is at about 70' ish IIRC), again covered with marine life, similar to the Northern Channel Islands.
Santa Barbara Island is really neat because it has sea lion rookeries, so the boats make a point of diving with the sea lions. SBI is also the home of about 80gazillion brittle stars. I'm not sure this is the season (think it is), but at one dive site, the bottom is covered as far as the eye can see with brittle stars - it looks like really long yellow shag carpet until you get up close and shine your light on all the gillions and gillions of multicolored brittle stars. Has kelp. Also some dives with shallow sloping sandy bottom where you see some big Halibut. Lots of bat rays at SBI (as well as everywhere else). Also electric rays (Torpedo Rays), although they are everywhere as well.
San Clemente Island is the farthest out and a big favorite because it has really nice topography, kelp forests and on days with really great viz, it can knock your booties off. Although I wouldn't know about that, because out of the 5 or so times I've been there, the viz has been standard CA viz - 20-40'. Those who love SC really love it though. Has nice kelp too.
Logistics: All these boat trips assume you have all your own gear, including tanks and weights. If you don't, several of the boats will rent gear, or you can book with one of the local dive shops (who do charter trips), and rent gear from them. Let me know if you'd like some phone numbers of some local shops. All of the boats, except for boats to Catalina or Anacapa, board the night before and sleep on the boat on the way out. Start diving in the morning, typically get 3 dives, then motor back in all afternoon. Anacapa and Catalina are day trips - Anacapa is 1.5 hours each way by boat, and Catalina aobut 2.5, so those boats usually leave at 6 or 7 in the morning and get home in the evening. You can often sleep on these boats the night before as well, in the harbor, if you don't want to be driving around at 5am in the dark looking for a boat you've never been on before. For sleeping on the boats you'll want a sleeping bag, although most offer blankets and pillows.
Oil Rigs. If you've never dove an oil rig, you should try to find a boat doing the rigs when you're out here. The Rigs out of San Pedro/Long Beach are really fun and can be really beautiful, depending on the viz, which varies. I've been with 5' viz and with 100'. Closer to 20-30ish is probably about average. Tons of inveribtrates encrusting the legs of the rigs, sea lions, and some more pelagic species including some stunning jellies often float by, although with your diving experience you've probably seen a ton of those before. The Oil Rigs trips can be harder to schedule, as the boats have to get special permission to go there and don't go that often, but if you get the opportunity, go! They're also only about a half hour from the harbor, so it's an easy day trip.
Shore Diving. We also have pretty good shore diving, and clubs that go quite often (vetdivers?). Somebody will have to give you more specific information, but there are alot of people who love the shore diving here. I prefer boats, so I don't have too much information, but if you do go, try to get somebody to take you to "Old Marineland".
I don't know much about San Diego area, except that there are plenty of charter boats you can get on (Lois Ann is one), and they do Wreck dives (really big good wrecks), Kelp dives, and trips that are a combo of the two. They also have plenty of really good shore diving, as your friend in Laguna will tell you (La Jolla shores, as well as spots in Laguna itself). And, it seems they currently have Humboldt Squid, if you're brave . . .
If I were you, I'd structure my trip around a multi-day trip on one of the Truth boats out of Santa Barbara. If you went on a 3 day trip, you'd get a pretty good flavor for the socal diving experience, and the logistics are easy as they rent gear and that saves you hotel nights.
Have fun!
Susan
Edited to add: Vazuw is right, expect water temperatures between 55 and 65. Most folks dive in 7mm full wetsuits with hoods, or else drysuits. Both can be rented locally. It's worth it though!
